I will replace the nutshell of this topic with a different, yet similar situation that happened last time:

My White Mage in Sohm Al is struggling and I supported him as Red Mage. So once in a while I casted Vercure to get our health above the 30% mark - this was the average health during the dungeon whenever I looked at the party member's health bars. The healer, a Lalafell, was clearly not able for some reason to keep us in good shape, neither was the tank as I also pulled aggro several times by going into melee. So I just stopped attacking, backed off and attacked the only enemy that was not orange in the emnity list and assisted the dedicated healer with these Vercures. But I never even remotely thought of any ways of "punishing" these players by acting irritating myself when it's clear they are struggling.

Is there an unwritten rule that says "Let my mates die despite I have the tools on my fingertips to prevent that?"

Sorry, but if you are a tank and you go like "You tank it, you die" then you should stop playing in groups in general.
What kind of attitude is that? Not playing your role is griefing (at least to GM Karrsul!), so you are also pretty much in trouble if you refuse to heal as healer or to take aggro as tank.

And please don't come around the corner with the infamous "I run into everytihng on purpose" players as these are absolutely rare. The majority are Black Mages with a flare too early or a bard using all of his stuff in quick succession etc.

Once I had a healer that would stop to heal me and my cousin for no apparent reason, probably with the same, weird "unwritten rule" attitude we might have broken in this guy's eyes. We did nothing wrong and our health refreshed inbetween the pulls, but it was just weird to see in action what happens when people take this nonsense too far. One time I, as Red Mage again, kept healing one poor soul as the healer showed the same disgusting attitude of "You spank, you tank" - the poor bard was running in circles while the other (cough)holes watched. That made me mad and kinda sad... Healer and tank watching and I try to assist fight and healing until my mana reached critical levels. Are we a team or what?

So, no, there are no unwritten rules. Just courtesy and common sense. Use it. You won't teach someone a lesson by letting him die on purpose. Be a real, honorable tank and help this guy out. For everything else is the votekick feature.

Sincerely,